Five related projects are described in this proposal. Each examines a particular cell surface or extracellular component to determine its role in early morphogenesis of the sea urchin. (1) Ectoderm-specific and endoderm-specific cell surface antigens have been described that have the following properties: they appear at or near the beginning of gastrulation and they influence the specificity of cell recognition in a germ layer-specific fashion. The goal will be to characterize these antigens functionally and biochemically. (2) Primary mesenchyme cells have the capacity to differentiate in vitro after being isolated at the 16 cell stage. The goal is to determine how mesenchyme cells leave the blastoderm, invade the blastocoel, and influence the organization of spicule skeletone. (3) Hyalin, an extracellular matrix protein, has been isolated biochemically, and localized throughout development by immunofluorescence. The studies suggest the existence of a cell surface hyalin receptor. The goal is to isolate the receptor and to study its role in gastrulation. (4) Ligatin, a small molecule with known baseplate function in other systems, has been identified in sea urchin membranes. The goal is to study ligatin and the stage-specific proteins associated with it for potential roles in morphogenesis. (5) Basement membranes have been isolated from sea urchin gastrulae. The goal is to study the biology of the basement membrane with respect to adhesive properties and the penetration of gastrulating primary mesenchyme cells.